New reference frame: SkyPlane

[ajtribick]

I have added a new reference frame for .ssc files, SkyPlane. This is essentially a shorthand for a TwoVector frame which is compatible with the orbital elements published for binary star and exoplanetary orbits. Previously using this frame would require either the use of Grant Hutchison's starorbs.xls spreadsheet to convert the elements into the ecliptic frame or the specification of a (somewhat verbose) TwoVector frame.

The following OrbitFrame definitions are equivalent:

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OrbitFrame {
    SkyPlane {
        Center "star"
        Target "barycenter"
    }
}

OrbitFrame {
    TwoVector {
        Center "star"
        Primary { Axis "z" RelativePosition { Observer "barycenter" Target "SSB" } }
        Secondary { Axis "x" ConstantVector { Vector [ 0 0 1 ] Frame { EquatorJ2000 { Center "star" } } } }
    }
}

The defaults for the Center and Target properties are the parent object and the top-level barycenter (this ensures that the orbit doesn't change orientation as the components in a multi-star system follow their orbits). Therefore in most cases, it will be possible to abbreviate the frame definition to just:

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OrbitFrame { SkyPlane { } }

This frame can be used as either OrbitFrame or BodyFrame. At present this only applies to objects defined in .ssc files, getting it to work in .stc files (where it would be really useful for binary star orbits) is something I will have to take a look at.

[ajtribick]

Following discussions in this thread, I've backed out this feature.

[Hungry4info]

A shame.

What was the problem with it? The discussion in that thread quickly rose above my ability to follow

[selden]

The problem is that different researchers use different standards for the SkyPlane coordinate system, and too often it is not obvious which one they used in their catalogs. As a result, transcribing published values often gives the wrong results -- with stars traveling in the wrong directions in their orbits, for example.

[Hungry4info]

Understood. I appreciate it.

So then it doesn't really matter if we use Grant's starorbs.xls document or a hypothetical SkyPlane system incorporated into Celestia.

[Fenerit]

Just curious here: the back out of the skyplane feature is related also to the issues arose in this thread?:
http://www.shatters.net/forum/viewtopic.php?f=3&t=15798

[ajtribick]

Just curious here: the back out of the skyplane feature is related also to the issues arose in this thread?:
http://www.shatters.net/forum/viewtopic.php?f=3&t=15798

I hadn't been following that thread, but no, this issue is not related to it.

So then it doesn't really matter if we use Grant's starorbs.xls document or a hypothetical SkyPlane system incorporated into Celestia.

Since the SkyPlane frame did the same job as starorbs.xls (with the same assumptions about the coordinate system), there is no difference between using one or the other.

Incidentally it looks like the extrasolar planets orbits we've got in Celestia are fine, because the incorrect choice of z-axis in the coordinate system is cancelled out by interpreting the value of ? as being that for the planet's orbit instead of (as per the usual convention for spectroscopic binaries) that of the reflex orbit of the primary star. Since we don't have to match the skyplane orbits of any of the exoplanets, as they are unknown, this works out fine.

[Hungry4info]

Indeed, the ease in defining exoplanet orbits was the main thing I was looking forward to with this hypothetical feature.